This invention relates to X-ray diffraction apparatus for measuring in-plane distribution of the interplanar spacing of the crystal lattice of an epitaxial thin film deposited on a single crystal substrate and a method for measuring X-ray rocking curves.
An X-ray rocking curve measurement using X-ray diffraction has often been used for measuring the lattice constants of a single-crystal thin film which is epitaxially deposited on a single crystal substrate. The crystal lattice constants can be calculated by using Miller indices and its interplanar spacing. In the X-ray rocking curve measurement, a diffraction peak angle is precisely measured so that the interplanar spacing can be calculated. The resulting interplanar spacing is used to determine the lattice constants of the crystal.
FIG. 14 is a plan view of the prior-art X-ray diffraction apparatus for measuring precisely in-plane distribution of the interplanar spacing of the crystal lattice of a sample. This X-ray diffraction apparatus is disclosed in xe2x80x9cX-ray Diffraction, Experimental Physics Course Vol. 20xe2x80x9d, edited by Kazutake Kohra, issued by Kyoritsu Shuppan, p. 477, 1988. This X-ray diffraction apparatus uses the double crystal method with which an X-ray diffraction rocking curve of a sample can be measured precisely. X-rays from an X-ray source 10 reflects at the first crystal 12, passes through the opening of a movable slit 14 and is then incident at a point P on a sample 28 which is the second crystal. The diffracted X-rays from the point P are detected by an X-ray detector 18 which may be, for example, a scintillation counter. The sample 28 is under xcfx89-rotation within a small range of angle. The intensity of the diffracted X-rays is detected by the X-ray detector 18 during the sample rotation, so that the rocking curve of the aimed diffraction peak can be obtained. The rocking curve is defined as a peak profile of diffraction which may be illustrated in a graph with an incidence angle of X-rays to the sample on the abscissa and an intensity of the diffracted X-rays on the ordinate. The diffraction angle of the sample can be obtained from the rocking curve, and the interplanar spacing of the sample can be calculated by the diffraction angle.
When the movable slit 14 is translated in a direction perpendicular to the X-ray beam, the irradiation point is shifted to a point Q on the sample 28 so as to obtain another rocking curve at Q. The X-ray irradiation pointon the sample 28 can be thus altered by the translation of the movable slit 14 and therefore a plurality of the rocking curves for the respective points on the sample can be measured one after another. After all, in-plane distribution of the interplanar spacing can be obtained based on the resulting many rocking curves.
The prior-art X-ray diffraction apparatus mentioned above has the following drawbacks:
(1) It takes a very long time to measure many rocking curves for respective points on the sample (i.e., an area map measurement) because the rocking curves are to be measured one after another for different points. If the resulting area map should be used for process control of the deposition for a uniform composition, the area map measurement should be completed in a short time. The prior art apparatus however requires a too long time to do such a process control.
(2) The opening of the movable slit 14 is usually about 0.5 to 2.0 mm in width and about 5 to 10 mm in height. The size of the irradiated region on the sample 28 is nearly equal to the size of the opening of the movable slit 14 because the X-rays travel parallel to each other from the first crystal 12 to the sample 28. The intensity of diffracted X-rays from the sample 28 are detected by the X-ray detector 18, noting that the detected intensity is the sum of X-ray intensities diffracted from the all points within the above-sized irradiated region on the sample 28. The interplanar spacing derived from the resulting rocking curve is therefore an average value for the above-sized X-ray irradiated region on the sample 28. If it is intended to measure the interplanar spacing for a narrower region than the above-sized standard irradiated region, the opening of the movable slit should be smaller than the above size. For selecting one of various sizes of the irradiated region, many movable slits 14 of different opening sizes must be prepared.
(3) The movable slit 14 should be movable two-dimensionally in its plane for measuring the in-plane distribution of the interplanar spacing, requiring a moving mechanism for the movable slit 14. The use of the movable slit 14 may be replaced by the movement of the sample 28 in its plane., requiring, in such a case, a mechanism for moving the sample two-dimensionally in its plane.
(4) For obtaining in-plane distribution of the interplanar spacing, means for detecting a position at which X-rays are incident on the sample is required.
(5) If it is intended to irradiate only a certain small region, e.g., of about several tens micrometers on the sample, an X-ray beam should be narrowed by a slit. Moreover, the slit should be disposed as close to the sample as possible because any divergence angle of X-rays causes a blur of the irradiated region, hence requiring a special-designed slit.
(6) If it is intended to irradiate only a certain small region on the sample, the narrowed X-ray beam should be directed precisely to the small region with accuracy of about several micrometers. Therefore, positioning means such as a microscope is required and the moving mechanism for the movable slit or the sample should be of high accuracy. Besides, the positioning work is troublesome.
Accordingly it is an object of the invention to provide X-ray diffraction apparatus which can measure in-plane distribution of the interplanar spacing of the crystal lattice of a sample in a short time.
It is another object of the invention to provide X-ray diffraction apparatus which can measure in-plane distribution of the interplanar spacing without a moving mechanism for translating a slit or the sample.
X-ray diffraction apparatus according to the invention has a crystal collimator system which reflects X-rays which has only a predetermined wavelength out of the X-rays generated by an X-ray source. The reflected X-rays are incident on a sample. X-rays diffracted by the sample are detected by a two-dimensional position-sensitive X-ray detector. The sample is under rotation around an xcfx89-axis which is parallel to the sample surface. X-ray intensities are detected and recorded at the same time for respective points of the detecting surface of the two-dimensional position-sensitive X-ray detector at every rotation angle of the sample with a predetermined pitch of angle, so that plural rocking curves can be measured at the same time for the respective points of the sample.
The crystal collimator system in the invention may have a single crystal plate or a plurality of crystal collimators combined with each other. The two-dimensional position-sensitive X-ray detector may be an X-ray CCD camera.
According to the invention, the two-dimensional position-sensitive X-ray detector is used for the precision measurement of lattice constants, so that a plurality of X-ray rocking curves can be measured at the same time for the respective points on the sample and an area map of the lattice constants of the sample can be obtained in a short time.